Soybeans (Glycine max) are a major cash crop and investment commodity around the world. Soybean oil is one of the most widely used edible oils, and soybeans themselves are used worldwide both in animal feed and in human food production.
Soybean cyst nematode (SCN) causes substantial yield loss in North American soybean. Heterodera glycines Ichinohe, was first identified on soybeans in the United States in 1954 at Castle Hayne, N. C. Winstead, et al., Plant Dis. Rep. 39:9-11, 1955. Since its discovery the soybean cyst nematode (“SCN”) has been recognized as one of the most destructive pests in soybean in the United States and worldwide. It has been reported in nearly all states in which soybeans are grown, and it causes major production problems in several states, being particularly destructive in the midwestern states. See generally: Calwell, et al., Agron. J. 52:635-636, 1960; Rao-Arelli and Anand, Crop. Sci. 28:650-652, 1988; Baltazar and Mansur, Soybean Genet. Newsl. 19:120-122, 1992; Concibido, et al., Crop. Sci., 1993. For example, susceptible soybean cultivars had 6-36% lower seed yields than did resistant cultivars on SCN race-3 infested sites in Iowa (Niblack and Norton 1992). Since the discovery of SCN in the United States in the 1970s, extensive efforts have been made to identify new SCN resistance sources by screening Glycine max plant introductions (PIs) of the USDA soybean germplasm collection (Anand et al. 1988; Arelli et al. 2000; Arelli et al. 1997; Young 1995). Chen et al. (2006) used bioassay to characterize over 120 SCN resistance soybean accessions with SCN races 3, −5, and −14 and reported many PIs including PI 437654, PI 438489B, PI 90763, PI 89772, PI 404198A, and PI 567516C with high resistance levels to multi-races.
Although several SCN resistance quantitative trait loci (QTLs) have been discovered in PI 437654 (Concibido et al., 2004, U.S. Pat. No. 6,096,944 issued to Vierling et al. and U.S. Pat. No. 6,538,175 issued to Webb), many SCN resistance QTLs remain to be identified. More SCN sources of resistance have been evaluated extensively to identify novel QTLs and epistatic effects between QTLs (Wu et al 2009). Among soybean PIs evaluated for SCN resistance, PI 437654, PI 467312, PI 438489B, and PI 567516C have been reported to be highly resistant to multi-races (also known as HG types) of SCN. In addition, PI 567516C is also resistant to a synthetic nematode population LY1 and genetically unique from most other SCN resistant sources, including Peking and PI 88788 that are widely used in current SCN resistant varieties.
SCN accounts for roughly 40% of the total disease in soybean and can result in significant yield losses (up to 90%). SCN is the most destructive pest of soybean to date and accounts for an estimated yield loss of up to $1 billion dollars annually. Currently, the most cost effective control measures are crop rotation and the use of host plant resistance. While breeders have successfully developed SCN resistant soybean lines, breeding is both difficult and time consuming due to the complex and polygenic nature of resistance. The resistance is often race specific and does not provide stability over time due to changing SCN populations in the field. In addition, many of the resistant soybean varieties carry a significant yield penalty when grown in the absence of SCN.
Although the use of nematocides is effective in reducing the population level of the nematode, nematocide use is both uneconomical and potentially environmentally unsound as a control measure in soybean production. Neither is crop rotation a practical means of nematode control, since rotation with a nonsusceptible crop for at least two years is necessary for reducing soybean losses. Therefore, it has long been felt by soybean breeders that use of resistant varieties is the most practical control measure.
Screening of soybean germplasm for resistance to SCN was begun soon after the discovery of the nematode in the United States, and Golden, et al. (Plant Dis. Rep. 54:544-546, 1970) have described the determination of SCN races. Although SCN was discovered in North America about 40 years ago, soybean breeding for resistance to SCN has mostly utilized genes from two plant introductions—Peking and PI88788, and while these lines have resistance genes for several SCN races, including race-3, they do not provide resistance to all known races.
The plant introduction PI 437.654 is the only known soybean to have resistance to SCN races-3 (Anand 1984) (Anand 1985) and (Rao-Arelli et al. 1992b). However, PI 437.654 has a black seed coat, poor standability, seed shattering, and low yield, necessitating the introgression of its SCN resistance into elite germplasm with a minimum of linkage drag. Conventional breeding with PI 437.654 produced the variety ‘Hartwig’ (Anand 1991), which is more adapted to cultivation and can be used as an alternative source of SCN resistance in soybean breeding programs.
Resistance to SCN is multigenic and quantitative in soybean (Mansur et al. 1993), though complete resistance can be scored qualitatively. For complete resistance to SCN, PI 437.654 has two or three loci for race-3, two or four loci for race-5, and three or four loci for race-14 (Myers and Anand 1991). The multiple genes and SCN races involved contribute to the difficulty breeders have in developing SCN resistant soybean varieties.
Breeding programs for SCN resistance rely primarily on field evaluations where natural nematode populations occur. However, these populations can be mixtures of undetermined races (Young 1982) and the environment can affect the overwintering and infection capability of the nematodes (Niblack and Norton 1992). Although evaluations using inbred nematode populations in controlled greenhouse environments are superior, they are prohibitively expensive and the nematodes are difficult to manage for large breeding programs (Rao-Arelli, pers comm). These deficiencies in each evaluation method make SCN resistance a difficult trait to manipulate in soybean improvement programs. Host plant resistance is an effective approach to control this pest; however, continuously growing the same resistant cultivar(s) may result in SCN population shifts and loss of SCN resistant phenotype.
Therefore, discovery of new sources of genetic resistance is fully warranted.